In February 1948 the Government brought forward a new Representation of the People Bill which removed the right of owners of business premises to a second vote; this would have had the effect of reducing the electorate of the City of London from 12,500 to 4,600. The Bill proposed also to end the City of London as a separate constituency and to merge it with the adjacent boroughs of Finsbury and Shoreditch.[6] During debates on the Bill, the Government amended it to substitute a link between the City of London and the City of Westminster.[7] In introducing the amendment the Home Secretary James Chuter Ede noted that the alterations to the constituencies in Westminster, Chelsea and Kensington had been agreed unanimously at a conference between the Members of Parliament and representatives of the boroughs affected.[8]

No alteration was made by the First Periodical Report on constituency boundaries in 1954.[9] In the Second Periodical Report in 1969, the Boundary Commission wrote that their initial feelings were that "except for a minor alteration to follow a new ward boundary" they felt that there was "no reason to disturb" the constituency, and they received no objections to this proposal. Westminster City Council later suggested that the constituency could be more accurately named as 'The City of London and Westminster South'; the Boundary Commission found opinion divided and left the name unchanged when it published revised proposals for two other constituencies within the City. Subsequent representations on the name were received and the Commission decided that, although justified on historical grounds, the name was "not now entirely accurate" and so proposed the renaming as suggested by the City Council.[10]

In initial proposals during the Third Periodical Review (1983), the Boundary Commission proposed to abolish the St Marylebone constituency and add four wards from it (Cavendish, Baker Street, Bryanston and Regents Park) to the previous City of London and Westminster South constituency; they provisionally named the result 'The City of London and Westminster'. After a local inquiry, the Regents Park ward was removed and Hyde Park ward (from the Paddington constituency) was added; unanimous opinion at the inquiry favoured naming the result 'The City of London and Westminster South'.[11]

For the Fourth Periodical Review (1995), the Boundary Commission paired the City of Westminster with the Royal Borough of Kensington and Chelsea for consideration. The Commission's initial proposals, to expand the constituency by two wards (Bayswater and Lancaster Gate) formerly in Westminster North and to return to the name 'Cities of London and Westminster', were upheld after a local inquiry, despite multiple counter-proposals.[12]

At the Fifth Periodical Review (in 2007), the initial proposals of the Boundary Commission paired the City of Westminster with the London Borough of Brent although they involved only minor changes to the Cities of London and Westminster constituency to take account of new ward boundaries. Widespread objections ("almost universal hostility") to the pairing led to a local inquiry, which decided that Westminster and the City of London should be reviewed separately and not paired with any other borough. The Commission proposed a new Cities of London and Westminster constituency in which the revised Bayswater and Lancaster Gate wards were removed.[13]

Early proposals made during the initial stages of the postponed Sixth Periodic Review of Westminster constituencies proposed linking the City of London to the southern wards of Islington in a constituency to be known as "The City of London and Islington South".[14] Most of the Westminster wards were proposed to form part of a Westminster and Kensington constituency.[15] This proposal was the first to suggest a split between the two Cities in Parliamentary elections since they were joined and proved unpopular in consultation; the Boundary Commission revised them to return the link between the City of London and the City of Westminster,[16] although the review was subsequently placed on hiatus.

In 2016 the Boundary Commission produced a second attempt at the Sixth Periodic Review. Its proposed Cities of London and Westminster comprises the City attached to Regent's Park and Abbey Road to the north-west, Knightsbridge/Belgravia to the west, and Holborn/Covent Garden to the north.[17]

1974-1983: The City of Westminster wards of Charing Cross, Churchill, Knightsbridge, Millbank, Regent Street, Victoria Street, and Warwick, and the City of London.

1983-1997: The City of Westminster wards of Baker Street, Belgrave, Bryanston, Cavendish, Churchill, Hyde Park, Knightsbridge, Millbank, St George's, St James's, Victoria, and West End, and the City of London.

1997-2010: The City of Westminster wards of Baker Street, Bayswater, Belgrave, Bryanston, Cavendish, Churchill, Hyde Park, Knightsbridge, Lancaster Gate, Millbank, St George's, St James's, Victoria, and West End, and the City of London.

2010-present: The City of Westminster wards of Bryanston and Dorset Square, Churchill, Hyde Park, Knightsbridge and Belgravia, Marylebone High Street, St James’s, Tachbrook, Vincent Square, Warwick, and West End, and the City of London.

The Cities of London and Westminster seat contains the two historical centres of the capital. The City of London is an international financial centre - while Westminster, home to the Houses of Parliament, Whitehall and Downing Street, represents Britain's political and cultural heart.

The seat includes iconic landmarks such as Buckingham Palace and St Paul's Cathedral, the West End's Theatreland and Soho. Some of the country's wealthiest residents live in exclusive Mayfair, Belgravia and Knightsbridge. Less than half the population was born in the UK - a fifth hail from Europe, while one in twenty is American, according to ONS 2011 Census figures for England and Wales.

The Westminster City Council wards are heavily represented by the Conservatives; in the most recent elections in 2018 the Labour Party won two seats in the Churchill ward in the constituency, and one in West End ward.

The constituency also incorporates the City of London council. This includes some of the lowest populated wards in the United Kingdom, such as the Coleman Street ward which has a total electorate of 2 and the Aldgate ward which has a total electorate of 27. [19]